(1) Field of the Invention
The present invention pertains to a direct voltage multiplier which can be integrated into a semiconducting structure, notably in order to control a power transistor made on a semiconducting substrate.
The growing number of electronics applications in all sectors of industry has led to a proliferation of electric cable lengths. For automobiles especially, the electrical wiring of the various mechanisms is becoming more and more complex, with each of these mechanisms being controlled practically on an individual basis. This wiring generally takes the shape of a harness of wires, of ever-increasing bulk, which it is practically impossible to inspect and repair after assembly. The solution to this problem lies in the use of multiplexing. Since each accessory is capable of recognizing the information that is relevant to it, the accessories can all be connected by one and the same control wire. A multiplexing arrangement of this type results in the association of a power transistor and its control logic on one and the same substrate. One of the problems raised by this new conception of automobile electronics relates to the control of a power transistor in series with a load. In particular, if the power transistor is of the VD MOS (vertical, diffused channel, field-effect transistor) type, the voltage applied at its gate to make it conductive must be greater than the voltage existing between its source and its drain. Since the VD MOS and its load are directly connected to the accumulator battery, there should therefore be a direct voltage which is greater than the battery voltage. Since introducing another source of direct voltage into an automobile is not desired, it is necessary to resort to a voltage multiplier.
(2) Description of the Prior Art
Voltage multipliers are known in the prior art, based on the charging and discharging of a capacitor with control provided by a clock signal. Devices of this type comprise at least two diodes and are made of discrete elements which, therefore, require connecting wires. Since it is sought to avoid using cabling wires as far as possible, the multiplier must be integrated into the same substrate as the VD MOS. To simplify the manufacturing process, it is desirable to make the logic circuit which controls the VD MOS directly in the substrate which acts as the drain for the VD MOS without any special insulation between the region that constitutes the VD MOS and the logic region. It then becomes possible to take advantage of the diffusing operations, which will be used to prepare the VD MOS, to integrate the diodes mentioned above into the substrate. For example, if the substrate is of the N type, the operation in which a P type region is diffused (in order to make the VD MOS channel) could be used also to implant a P type well forming the anode of a diode. The N type diffusion, needed to form the source of the VD MOS, could then be used also to implant an N region, forming the cathode of the diode, in the P well. This solution, which seems attractive, cannot be considered. For it would be impossible to obtain a voltage, greater than the battery voltage, at the VD MOS gate owing to the junction set up between the P well of the diode and the N type substrate which is itself carried to the potential of the battery. One solution to this problem lies in insulating the diode thus created from the substrate. An insulation of this type implies special operations (such as the creation of a buried layer, guard rings, etc.) which cannot be performed at the same time as the VD MOS is being prepared. These additional stages would substantially increase the cost of the semiconducting device
In order to reduce these difficulties, the invention proposes to replace the diode driving the gate of the VD MOS by a resistor made from a depletion-mode MOS transistor and to modify the pulse ration of the clock signal so that this resistor behaves like the diode of a conventional multiplying circuit.
It must be noted that the voltage multiplier of the invention has been conceived to resolve a problem pertaining to a particular semiconducting structure. However, the new electrical pattern thus perfected can be used in other applications where a conventional structure would not be appropriate.